1. Field of the Invention
The invention relates to a clock-signal regenerator, comprising a crystal oscillator which can be frequency controlled and is incorporated in a phase-locked loop, in which regenerator the oscillator frequency is pulled by means of at least one load capacitance, coupled to the oscillator crystal, which crystal oscillator comprises a switching arrangement connected to a load capacitance for effectively connecting the load capacitance to the crystal in dependence on a control signal to be applied to the switching arrangement, which control signal represents the phase deviation between the clock signal to be regenerated and the regenerated clock signal.
The invention likewise relates to a voltage-controlled crystal oscillator suitable for use in a similar clock signal regenerator, and a telecommunication terminal comprising a similar clock signal regenerator.
2. Description of the Related Art
A frequency-controlled crystal oscillator of the type mentioned above is known from the U.S. Pat. No. 3,603,893.
It is advantageous to be able to pull the frequency of the crystal oscillator over a wide range. When applied in a clock regenerator the local oscillator is able to follow the clock signal to be regenerated over a wide frequency-range, which increases the practicability. Alternatively, it is possible in a given variation range of the clock signal to be regenerated, to apply a crystal having a larger frequency manufacturing-tolerance. The resulting possibly larger deviation between the incoming clock signal and the nominal oscillator frequency can then be adjusted by the greater frequency pulling.
The crystal oscillator known from the aforesaid United States Patent comprises a load capacitance (U.S. patent reference numeral 37) connected to the oscillator crystal and a switching arrangement formed by a field effect transistor (U.S. patent reference numeral 36) for short circuiting or effectively connecting this load capacitance to the crystal. By selecting a desired, large value of this capacitance, a desired large frequency pulling range can be achieved.
The advantage of a large frequency-pullng range is for example shown by the fact that when manufacturing the crystal oscillator tuning procedure for the nominal frequency can be dispensed with.
The switching arrangement in the above U.S. Patent is designed in the form of a semi-conductor switch element. Such switch elements can be controlled by fast change-over signals whose rise time is of the same order as or shorter than the oscillator frequency; alternatively it is possible to have the change-over controlled by slow change-over signals whose rise time exceeds (by far) the oscillator frequency.
In the event of fast change-over signals, pulses in the oscillator signal may occur owing to the fast change-over. If the polarity of these pulses is opposite to the instantaneous polarity of the oscillator signal, this may lead to a disturbance of the clock frequency derived from the oscillator. This is obviously an undesired effect. This problem could be solved by a synchronising arrangement, synchronising the change-over signals and the oscillator signal; however, such a synchronising arrangement constitutes an additional cost factor. In the event of slow change-over signals, however, the switch element forms a resistive load for the oscillator crystal during the change-over, as a result of which the oscillator may break down.